Подскажите, почему watch не срабатывает? Выдает ошибку : Error in callback for watcher "form": "TypeError: Cannot read property 'call' of undefined"

Необходимо отслеживать изменения в объекте формы, полей много, приведу маленький кусочек.

<b-form-input
            id="id-repo-label"
            v-model="
              form.SPCEX_DOC.document.selection_info.selection_info_rec.finstr
            "
            size="sm"
            plaintext
          >

</b-form-input>

data() {    
 return {
      dateFormatOptions: {
        year: "numeric",
        month: "numeric",
        day: "numeric"
      },
      isDateState: null,
      dateConstraints: {
        min: minDate,
        minRepayDate: minRepayDate
      },

      form: {
        SPCEX_DOC: {
          receiver: {
            addressee_info: {
              name: "СПВБ",
              id: 263
            }
          },
          sender: {
            addressee_info: {
              name: "Комитет финансов СПб",
              id: 302
            }
          },
          document: {
            ver: "3.03a",
            form: "selection_info",
            selection_info: {
              executor_phone: 78128965423,
              selection_info_rec: {
                initiator_code: 302,
                period: "",
                paying_date: today,
                booking_time_finish: "16:00",
                selection_id: 5628,
                min_ord_rate: 198,
                total_sum: 31201,
                finstr: "SPRA---RS-",
                selection_type: "RA",
                booking_time_start: "14:04:59",
                selection_date: today,
                initiator_name: "Комитет финансов СПб",
                currency: "RUB",
                repaym_date: minRepayDate,
                comment: ""
              },
              records: 1,
              executor_name: "executor_name1"
            },
            remarks: "Объявление об отборе заявок"
          },
          doc_props: {
            delivery_confirmation: "N",
            sender_doc_id: "sender_doc_id1",
            doc_ver: 750.952,
            doc_name: "doc_name1",
            doc_date: "2020-03-03",
            doc_type: "selection_info",
            delivery_mode: "P",
            doc_time: "14:04:59"
          }
        }
      },
      currency: {
        options: [{ value: "RUB", text: "RUB" }]
      },
      idRepo: {
        selected: "SPRA---RS-",
        options: [{ value: "SPRA---RS-", text: "SPRA---RS-" }]
      },
      show: true,
      boxTwo: ""
    };
  }
},
  watch: {
    // следим за изменением формы
    form: {
      deep: true,

      heandler: function() {
        console.log("changed");
      }
    }
  },

[Vue warn]: Error in callback for watcher "form": "TypeError: Cannot read property 'call' of undefined"